1. Field of the Invention
This invention relates to heat exchange devices, particularly heat exchange devices disposed in refinery or petro-chemical operations that are subject to fouling. The invention especially relates to the design of a pre-heat train upstream of a crude oil distillation operation.
2. Discussion of Related Art
Fouling is generally defined as the accumulation of unwanted materials on the surfaces of processing equipment. In petroleum processing, fouling is the accumulation of unwanted hydrocarbon-based deposits and inorganic deposits, such as salts and products of corrosion reactions, on heat exchanger surfaces. It has been recognized as a nearly universal problem in design and operation of refining and petrochemical processing systems, and affects the operation of equipment in two ways. First, the fouling layer has a low thermal conductivity. This increases the resistance to heat transfer and reduces the effectiveness of the heat exchangers. Second, as deposition occurs, the cross-sectional area is reduced, which causes an increase in pressure drop across the apparatus and creates inefficient flow in the heat exchanger.
Fouling in heat exchangers associated with petroleum type streams can result from a number of mechanisms including chemical reactions, corrosion, deposit of insoluble materials, and deposit of materials made insoluble by the temperature difference between the fluid and heat exchange wall. One of the more common root causes of rapid fouling, in particular, is the formation of coke that occurs when crude oil asphaltenes are overexposed to heater tube surface temperatures. The liquids on the other side of the exchanger are much hotter than the whole crude oils and result in relatively high surface or skin temperatures. The asphaltenes can precipitate from the oil and adhere to these hot surfaces. Prolonged exposure to such surface temperatures, especially in the latter section of the pre-heat train, or the so-called late-train exchangers, allows for the thermal degradation of the asphaltenes to coke. The coke then acts as an insulator and is responsible for heat transfer efficiency losses in the heat exchanger by preventing the surface from heating the oil passing through the unit. To return the refinery to more profitable levels, the fouled heat exchangers need to be cleaned, which typically requires removal from service, as discussed below.
Heat exchanger in-tube fouling costs petroleum refineries hundreds of millions of dollars each year due to lost efficiencies, throughput, and additional energy consumption. With the increased cost of energy, heat exchanger fouling has a greater impact on process profitability. Petroleum refineries and petrochemical plants also suffer high operating costs due to cleaning required as a result of fouling that occurs during thermal processing of whole crude oils, blends and fractions in heat transfer equipment. While many types of refinery equipment are affected by fouling, cost estimates have shown that the majority of profit losses occur due to the fouling of whole crude oils and blends in pre-heat train exchangers.
The pre-heat train is particularly susceptible to fouling, and loss of efficiency of heat transfer at this stage impacts the entire operation. It could be estimated that fouling and corrosion of heat exchangers in the crude pre-heat train leading to the furnace for the crude distillation unit exacts an energy penalty of greater than 100 MBtu/hr in a typical 100 kBD crude train. Furnace firing often limits throughput in crude units and coking units so the effects of fouling can be felt downstream as well.
Currently, most refineries practice off-line cleaning of heat exchanger tube bundles by bringing the heat exchanger out of service to perform chemical or mechanical cleaning. The cleaning can be based on scheduled time or usage or on actual monitored fouling conditions. Such conditions can be determined by evaluating the loss of heat exchange efficiency. However, off-line cleaning interrupts service. This can be particularly burdensome for small refineries because there will be periods of reduced or non-production.
Rigorous cleaning and hardware solutions can be costly and offer limited advantages over existing approaches. One existing approach utilizes carbon steel (CS) or chromium (Cr) containing ferritic steels, such as 5Cr and 9Cr, which can produce marginal benefits over standard exchangers. Other approaches have been attempted such as providing protective metal oxide films on surfaces susceptible to fouling. Known approaches vary in effectiveness and economic viability. Since the pre-heat train can include many heat exchangers, replacing or treating every heat exchanger in the pre-heat train can be extremely costly and may not yield sufficiently high mitigation results to justify the added expense.
Attempts have also been made to use vibrational forces to reduce fouling in heat exchangers. The basis for using vibration is to provide a mechanism by which motion is induced in the liquid in the tubes to disrupt the formation of deposits on the surface of the heat exchanger.
Mitigating or possibly eliminating fouling of heat exchangers can result in huge cost savings in energy reduction alone. Reduction in fouling leads to energy savings, higher capacity, reduction in maintenance, lower cleaning expenses, and an improvement in overall availability of the equipment.
There is a need to develop additional methods for reducing the effects of fouling and increase energy recovery. There is also a need for developing a system that is economically viable, especially in large applications such as refineries with many heat exchangers.